The phylum Porifera or sponges are classified as the most primitive group of multicellular animals. Their potential immunocompetence has yet to be decisively investigated. Immune reactivities at this level of phylogeny could provide important insights in understanding immunologic mechanisms in higher animals. We have found that the Hawaiian species, Callyspongia diffusa, is a most promising experimental animal because (a) it thrives in running sea water tanks in a controlled microcosm environment, (b) it invariably accepts intracolony isografts with rapid healing, and (c) it consistently rejects intercolony allografts in a manner strongly suggestive of acquired or adaptive immunity. Further studies would focus on the following questions: (1) Allogeneic specificity and possible immune memory detectible on a quantitative basis by controlled tissue transplantation, parabiosis and cell-mediated immunty experiments in vitro. (2) Mechanism of alloimmune recognition, and subsequent cytotoxic reactions to be characterized by histopathologic and electron microscopic techniques as well as by direct testing of isolated macromolecules and cellular interactions in vivo and in vitro. (3) Extent of antigenic polymorphism within and among populations of Callyspongia tested from separate localities in Kaneohe Bay, Pearl Harbor and elsewhere in Hawaii by appropriate tissue grafting. Characterization of strong versus weak histocompatibility barriers and possible existence of a major histocompatibility complex.